There is a class of semiconductor devices the members of which are commonly termed transferred electron devices. Perhaps the first of such devices to be developed and understood is now commonly termed the Gunn diode. This device exhibits a negative differential resistance, i.e., the current decreases as the field increases, and is important in microwave applications and has been used as, for example, a local oscillator or power amplifier. This effect, i.e., negative differential resistance, arises in semiconductors which have a high mobility, low energy minimum and a low mobility, high energy minimum in the conduction band. The two minima are commonly referred to as the lower and upper valleys, respectively. As the electric field within the bulk semiconductor is increased, a point is reached at which some electrons are scattered from the high mobility, lower valley to the low mobility, upper valley and, as a result, the average carrier drift velocity is reduced as the electric field is increased after scattering to the upper valley commences. Hence, the current through the semiconductor decreases after this point because of the decrease in average carrier mobility even though the voltage or field is increased. The negative differential resistance leads to, for example, the generation of coherent microwave radiation at a frequency between 1 GHz and 100 GHz.
It will be readily appreciated by those skilled in the art that the term "transferred electron," as applied to devices such as the Gunn diode, refers to a transfer of electrons, not in real space, but in momentum space. The transferred electron effect results from bulk semiconductor properties.
More recently, real space transferred electron devices have been proposed. In these devices, the electrons are physically transferred from one portion of the device to another. In these devices, the transferred electron effect results from the device structure. For example, Hess et al proposed such a device in Applied Physics Letters, 35, pp. 469-471, Sept. 15, 1979. The structure described comprised, what is now termed by those skilled in the art, a modulation-doped structure. It comprised layers of highly doped AlGaAs which were sandwiched between layers of lightly doped or undoped GaAs. The carriers in the high bandgap AlGaAs layers reached their minimum energy level in the low bandgap GaAs layers which, because of the reduced impurity scattering, exhibit high mobility. Hess et al observed that when a high electric field was applied parallel to the interface, the ensemble of electrons in the GaAs layers was heated to relatively high temperatures. An ensemble of electrons is a group or collection of electrons. The carriers in the low mobility layers were not so heated since the velocity acquired by these carriers is proportional to the mobility which is low. Some of the resulting heated, i.e., nonthermal, carriers in the GaAs layers enter the AlGaAs layers by thermionic emission, and combine there with donor atoms. As a result, the current through the channel formed by the GaAs layers begins to decrease, and the structure exhibits negative differential resistance.
See also Applied Physics Letters, 38, pp. 36-38, Jan. 1, 1981, and Applied Physics Letters, 40, pp. 493-495, Mar. 15, 1982. The former paper reported current-voltage characteristics of GaAs/AlGaAs heterojunctions which the authors believed demonstrated real space electron transfer. The latter paper described a microwave oscillator. U.S. Pat. No. 4,257,055, issued on Mar. 17, 1981, described two- and three-terminal switches as well as a photodetector using the structure first described by Hess et al.
However, consideration of these devices leads one skilled in the art to realize that transferred carriers are trapped. Consequently, when the voltage is reduced, the current does not decrease as rapidly as would be desired because of the charge storage. As a result, the devices will be relatively slow.
Keever et al describe both switching and charge storage in IEEE Electron Device Letters, EDL-3, pp. 297-300, October 1982. The authors claimed that the carriers were transferred from a GaAs layer having a high electric field and hot electrons through an AlGaAs layer to a GaAs layer having a low electric field and cold electrons. The only effect that can be expected in this device is carrier storage in the "cold" GaAs layers which can lead to a negative differential resistance and memory effects. However, this device cannot be used for microwave generation or amplification since the return of electrons from the cold layers to the hot layers is slow. If the cold layer is electrically contacted, there will be no charge storage and no negative differential resistance. Storage effects on the scale of 1 minute observed by Keever et al are likely due to trapping in the AlGaAs layers, rather than storage in the cold layers since it appears difficult to effect electrical insulation of the side and main contacts of the depicted device.